In a power supply that supplies power to a Radio Frequency (RF) power amplifier, it is commonplace to employ a voltage modulation in the power supply.
Hereinafter, the voltage modulation will be described by taking the RF power amplifier as a load. To deal with the increasing user requirements for bandwidth, the modulation mode for the communication system becomes more and more complicated. One prominent problem is that the efficiency of the RF power amplifier is low, which is the bottleneck for improving the efficiency of whole communication system. As to the linear power amplifier, if a conventional DC power supply is used, to preserve linearity, it is required that the supply voltage be greater than the peak voltage of RF signals. When the peak of RF signals is relatively low, the power amplifier simultaneously withstands higher voltage and load current, and thus the efficiency of the power amplifier is relatively low. The average efficiency of the power amplifier depends on the Peak to Average Power Ratio (PAPR) of the RF signals. To maximize communication bandwidth within a limited frequency band, modern communication systems employ a modulation mode with a non-constant envelope (amplitude) and higher PAPR. For example, the PAPR is 6.5 db˜7.0 dB in the Wideband Code Division Multiple Access (WCDMA) system and the PAPR is 9.0 db˜9.5 dB in Orthogonal Frequency-Division Multiple Access (OFDMA) used in the Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMax), which causes a reduction of the efficiency of the power amplifier. A series of other problems appear, such as increase of the power amplifier's size and weight, and higher requirements to thermal environments for the air cooling, which results in increased application and maintenance costs. Therefore, it is significant to improve the efficiency of the power amplifier.
According to the existing literature and technologies, the manner for improving the power amplifier depending on the power supply technology mainly comprises the Envelope Elimination and Restoration (EER) power supply and the Envelope Tracking (ET) power supply. According to the EER power supply technology, a RF signal to be amplified is separated to an envelope and a phase modulation signal utilizing the feature i.e., a constant envelope signal may be effectively amplified by a nonlinear power amplifier, then the nonlinear power amplifier is powered by the ET power supply to restore the amplified RF signals. Since the amplified signal amplitude depends on the output voltage amplitude of the ET power supply, it requires higher tracking precision of the ET power supply, otherwise the linearity of the amplified signal will be affected. The ET power supply technology employs the linear power amplifier, and improves the efficiency of the linear power amplifier through dynamically adjusting the supply voltage of the ET signals. In the above two solutions, dynamically modulating the output voltage of the power supply is required. The power supply modulator must simultaneously keep a high efficiency to ensure that the two solutions may effectively improve the efficiency of the whole amplifier system.
In a modern communication system, an RF envelope signal has a wider bandwidth. For example, the bandwidth of WCDMA with a single carrier is 5 MHz, and the bandwidth of WCDMA with four carriers is 20 MHz. The ET power supply is required to provide a high modulation bandwidth and high efficiency. In the prior art, a switched-mode power supply regulator is able to provide a high conversion efficiency. However, extremely high switching speed is required in the application for 20 MHz bandwidth, which cannot be achieved through the conventional switch devices, which causes the reduction of the conversion efficiency of the regulator. In the prior art, the switched-mode power supply regulator always cooperates with the linear power supply regulator to utilize the high frequency feature of the linear power supply regulator and the efficiency of the switch-mode power supply regulator, so as to optimize the precision of modulation and efficiency. A typical structure is shown in FIG. 1, in which a linear regulator 201 employs a feedback control of an output voltage 207, so as to ensure that the output voltage 207 of the linear regulator 201 tracks a reference input signal 206. A switch regulator 102 is a current source structure which is a composed of a step-down switch circuit, i.e., a BUCK circuit. The control manner of the switch regulator 102 is that the linear regulator 201 outputs current 208 to a hysteresis controller 103. That is, detecting the output current 208 of the linear regulator 201, when the output current 208 of the linear regular 201 is high, a switch tube 104 turns on and the output current of a switch regulator 102 is increased; when the output current 208 of the linear regular 201 is low, a switch tube 105 turns on and the output current of the switch regulator 102 is reduced; thereby, controlling the amplitude of the output current 208 of the linear regulator 201 be in a low range and reducing the output power the linear regulator 201. Since the efficiency of the linear regulator is relatively low, the reduction of the output power of the linear regulator 201 may help to improve the efficiency of the power supply modulator system.
However, in the prior art above mentioned, there are the following issues: the current change rate of the switch regulator 102 with the BUCK circuit is fixed, which cannot adapt to various load current change rates. Taking the RF power amplifier as an example, the current change rate of the RF envelope signals is relatively high, and the load current of the power supply regulator also changes therewith. The fixed output current change rate of the switched regulator may cause the tracking sometimes to fail under the current change rate of a higher load current; and may cause frequent switching under the current change rate of a lower load current, so that the switching frequency and the switching loss are increased and the system efficiency is reduced.